1) Field of the Disclosure
The disclosure relates to segmented copolymers, and in particular, to polydimethylsiloxane (PDMS)-urethane/urea segmented copolymers.
2) Description of Related Art
Elastomeric materials are required in a broad variety of applications including use in low temperature environments. However, many known elastomers do not maintain their advantageous elastic properties at low temperatures because such known elastomers may comprise polymer species that cause the elastomers to stiffen and become brittle below their glass transition temperatures (Tg). In addition, known polymers such as polysiloxanes and polyfluoroethers that are able to individually maintain flexibility at low temperatures can have reduced mechanical strength at low temperatures.
Segmented polysiloxane-urea copolymers are known, such as those disclosed in U.S. Pat. No. 7,363,360 to Yilgor et al. However, Yilgor et al. requires that a polyether segment be incorporated between the siloxane and urea hard segments in order to improve mechanical properties. Such an intermediate polyether segment introduces a structural element into the polymer system with a higher glass transition temperature (Tg) of approximately −50° C. that can reduce the low temperature range over which the material is elastic.
Elastomeric materials that possess the ability to demonstrate a high degree of toughness through both high elongation and tensile strength find application in a variety of environments. One example of this is the aerospace environment where low temperatures (less than −50° C.) are common. There is a need for materials that can maintain classic elastic properties and continue to demonstrate high degrees of toughness in these demanding environments. Many known elastomeric materials can undergo an abrupt increase in modulus of many orders of magnitude greater than −50° C. as temperature is lowered. This is due to the polymeric components that make up these materials possessing glass transition temperatures above the environmental temperature where such materials would find application.
It is desirable for elastomeric materials to be able to be formulated into a form that can be conveniently prepared and applied in the field. This can require precursor components that are capable of being stored and stable over acceptable time periods. Once activated, the elastomeric materials may demonstrate an acceptable pot life or be suspended in a medium that allows for convenient application. Low general toxicity of individual components, chemical resistance to fluids commonly found in aerospace environments, and the ability to be compounded with filler materials are also desirable.
Elastomeric materials and systems typically have chains with high flexibility and low intermolecular interactions and either physical or chemical crosslinks to prevent flow of chains past one another when a material is stressed. For an elastomeric material or system to demonstrate good elastic behavior at low temperatures, it is desirable that it be composed of elements that have low glass transition temperatures. Materials with low glass transition temperatures can have highly flexible chains with less flexible interchain interactions. Examples of polymer materials that have low glass transition temperatures are PDMS and fluoroethers. However, these materials may have reduced mechanical properties due to the fact that they are well above their Tg under ambient conditions. In order to compensate for this, silicone based materials and systems are known but may be formulated with fillers and heavily crosslinked to bring the mechanical properties of the final form to an acceptable level. Heavy crosslinking in PDMS based materials and systems can result in an increase in Tg from −120° C. associated with the linear chains due to crystallization effects and narrowing of the low temperature range over which the material is elastic. Thus, an alternative method to covalent crosslinking for reducing chain mobility in siloxane based elastomer systems is desired in order to maintain flexibility in these systems at low temperature.
One known method available to produce a physically crosslinked elastomer is the use of segmented polyurethane or urea systems. These species demonstrate strong hydrogen bonding potential between them and as a result can create strong associative forces between the chains. In order to produce elastomeric materials, regions of highly flexible and weakly interacting chains must be incorporated with strongly associating elements. This can be accomplished using a segmented copolymerization scheme. Segmented copolymers provide a straightforward synthetic route toward block architectures using segments with vastly differing properties. The end result of such synthesis are typically chains that possess alternating hard and soft segments composed of regions of high urethane bond density and the chosen soft segment component (e.g., siloxane), respectively. This covalent linkage of dissimilar hard and soft blocks drives the systems to microphase separation and creates regions of flexible soft blocks surrounding regions of hard blocks. The associative forces among the hard segments prevent flow under stress and can produce elastomeric materials capable of displaying high elongation and tensile strength.
PDMS as a soft segment component in a segmented polyurethane urea system for an elastomeric material that demonstrates low glass transition is known. In addition, silicone-urethane/urea systems are known. It is further known that differences in solubility parameter between PDMS and urea/urethane segments can provide a strong driving force toward microphase separation. Such sharp interfaces created in these materials can result in low values of elongation and tensile strength. Known methods exist to reduce the sharpness of the transition between the hard and soft segments in siloxane-urea materials by incorporating a second soft segment that can act as an interface creating a gradient between the two highly dissimilar materials. For example, polypropylene oxide (PPO) segments have been incorporated into segmented siloxane-urea materials in order to improve mechanical properties. However, while the PPO segments may increase tensile strength and elongation, the PPO introduces a second glass transition temperature into the overall material which can be much higher than the glass transition temperature of PDMS greatly reducing the low temperature range over which the material is elastic.
Moreover, known filled elastomer based coatings, gap fillers and sealants can lack robust durability, especially under environmental extremes such as low temperatures, (e.g., −50° C.).
Accordingly, there is a need for segmented elastomeric copolymer compositions and methods that provide advantages over known compositions and methods.